Nanoco Technologies

Creating warm white light with quantum dots (QD-SSL)

The white light LED market is hugely important, with the promise of increased lamp lifetimes and efficiencies paving the way for a revolution in the lighting industry. Colour rendering and efficiency are the two most important criteria for traditional light sources for general lighting. Lamp colour is typically specified according to the CIE 1931 chromaticity diagram (see below).

The ability of a light source to illuminate an object, such that the objects true colour is reflected, is denoted by its colour rendering index. For example, sodium lamp street lighting has poor colour rendering capability, making it difficult to distinguish a red car from a yellow car.

Current white light LED technology utilizes a cerium doped YAG:Ce (yttrium aluminium garnet) downconversion phosphor pumped by a blue (450nm) LED chip. The combination of blue light from the LED and a broad yellow emission from the YAG phosphor results in white light. Unfortunately this white light often appears somewhat blue, and is described as a cold or cool white. QDs are suitable for use as LED downconversion phosphors because they exhibit a broad excitation spectrum and high quantum efficiencies. Furthermore the wavelength of the emission can be tuned completely across the visible region simply by varying the size of the dot or the type of semiconductor material. As such they promise the generation of virtually any colour, and more importantly, warm whites which are strongly desired by the lighting industry. Additionally, by using a combination of up to three different types of dots with emission wavelengths corresponding to green, yellow, and red, it is possible to achieve white lights of different colour rendering indexes (see charts below).

Besides white lighting for general illumination there are other opportunities for QD-LEDs. For example, green LEDs are not particularly efficient, thus green-emitting QDs on top of an efficient blue LED chip may be a solution. Similarly, amber LEDs suffer from temperature dependencies and thus a QD solution may be applicable. Furthermore, because of the widely tunable QD emission, it is possible to have near UV pumped QD-LEDs, with combinations of QDs which emit virtually any colour on the chromaticity diagram. This could have important applications in signage, replacing neon bulbs etc.

References
a. H.-S. Chen, C.-K. and Hsu, H.-Y. Hong, IEEE Photonics Tech. Lett., 2006, 18, 193-195;
b. S. Nizamoglu, T. Ozel, E. Sari and H. V. Demir, Nanotechnology, 2007, 18, 065709;
c. H. Song, S. Lee, Nanotechnology, 2007, 18, 255202;
d. M. Ali, S. Chattopadhyay, A. nag, A. Kumar, S. Sapra, S. Chakraborty and D. D. Sarma, Nanotechnology, 2007, 18, 075401;
e. S. Nizamoglu and H. V. Demir, J. Opt. A: Pure Appl. Opt., 2007, 9, S419-S424.

A mixture of two colours produces a new colour whose xy coordinates fall on the line between their respective xy coordinates. A mixture of three colours produces a new colour whose xy coordinate falls within a triangle, whose vertices correspond to the xy coordinates of the three independent colours. The location of the coordinates of the mixed colour will depend on the relative intensities of the source colours.

White light LED spectra

a. YAG:Ce white light LED
b. dichromatic-single QD LED
c. trichromatic-dual QD LED
d. quadchromatic-triple QD LED

NB all spectra have 1931 CIE xy coordinates of 0.311, 0.324
colour rendering index increases from b to d